Causes of Low Back Pain

Pain Categories

Nociceptive (Mechanical) Causes

• The most common form of mechanical LBP
• Muscles and fascia are densely innervated with nociceptors and bear significant load
• Active trigger points show elevated substance P, bradykinin, norepinephrine, inflammatory cytokines (TNF-α, interleukins), and lower pH compared to normal muscle
• Presents with focal/diffuse tenderness, limited range of motion, increased muscle tension, and a normal neurological exam
• Acute muscle strain is often associated with a clear inciting injury; spasm may be secondary to deeper pathology

• Disk degeneration accounts for 26–42% of axial LBP
• In healthy disks, only the outer annulus fibrosus is innervated, but with degeneration, neoinnervation extends into the nucleus pulposus, creating new pain generators
• Disk degeneration also predisposes to facet joint degeneration (concomitant pain generators)
• Presents as axial back pain, often aggravated by flexion, sitting, and loading

• Found in up to 35% of LBP patients
• Frequently associated with arthritis or injury to the lumbar facet joints
• Pain may be unilateral or bilateral, occasionally radiating to the thigh but not below the knee
• Worsened by extension and lateral rotation toward the painful side
• Negative straight leg raise; normal neurological exam (unless coexistent radiculopathy)
• MRI/CT findings of facet arthropathy do not reliably correlate with clinical symptoms

• A major source of LBP
• Pain typically unilateral, may radiate to the hip or thigh
• Worsened by climbing stairs; positive Patrick test or single-leg standing test
• Managed with NSAIDs; corticosteroid injections provide temporary relief

Radicular (Neuropathic) Causes

• Annual incidence ~1%; point prevalence 1.5–4% (symptomatic); asymptomatic herniation prevalence 29–43%
• Common inciting events: falls, lifting, motor vehicle collisions
• L4–5 and L5–S1 are the most commonly affected levels
• Presents with LBP radiating into the lower leg in a dermatomal pattern:
  • L5 involvement: pain/numbness radiating to the dorsomedial foot; weakness of tibialis anterior
  • S1 involvement: pain/numbness to the lateral foot/small toe; weakness of peroneus longus/brevis
• Positive straight leg raise test (sensitivity ~80% for L5 and caudad roots)
• EMG/NCV may not always detect radiculopathy

• Affects ~11% of the U.S. population; prevalence increases dramatically with age
• Caused by: bulging/herniated disks, facet joint hypertrophy, spondylolisthesis, ligamentum flavum buckling/hypertrophy
• Hallmark symptom: neurogenic claudication — back pain radiating into the legs, exacerbated by walking/extension, relieved by sitting or leaning forward ("shopping cart sign")
• Most commonly at L4–5 (92%) and L3–4 (66%)
• Other features: wide-based gait, poor balance, diminished vibratory perception

• Stress fracture of the pars interarticularis
• Most common cause of LBP in adolescent athletes
• Associated with sports involving forceful hyperextension (gymnastics, American football)

Red Flag Causes (Serious / Systemic)

Risk Factors for Chronic LBP

• Female sex
• African-American race
• Older age
• Unemployment
• Obesity
• Sedentary lifestyle
• Psychopathology (depression, anxiety, poor coping, catastrophizing, PTSD)
• Greater initial disease burden, poor job satisfaction, secondary gain

Causes of Low Back Pain

Overview

"Back pain is a common complaint and a major cause of disability. Lumbosacral strain, degenerative disc disease, and myofascial syndromes are the most common causes."

1. Paravertebral Muscle & Lumbosacral Sprain/Strain

• Accounts for 80–90% of LBP reports
• Caused by lifting heavy objects, falls, or sudden abnormal spinal movements
• Sprain = well-defined acute injury; Strain = chronic, repetitive minor injuries
• Produces reflex muscle spasm, dull aching pain ± radiation to buttocks/hips
• Usually self-limiting, resolving in 1–2 weeks

2. Buttock Pain (related causes)

• Coccydynia: Trauma to the coccyx or surrounding ligaments
• Piriformis Syndrome: Buttock pain ± sciatic nerve distribution numbness/tingling; treated with trigger point injections
• Spinal stenosis and disc disease can also produce buttock pain

3. Degenerative Disc Disease (Discogenic Pain)

• Intervertebral discs bear at least one-third of spinal column weight
• The nucleus pulposus degenerates and becomes fibrotic with age and trauma
• Two major mechanisms of pain:
  1. Herniation of nucleus pulposus posteriorly into the spinal canal
  2. Loss of disc height → reactive osteophyte (spur) formation
• Most commonly affects the lumbar spine (greatest motion; thinnest posterior longitudinal ligament at L2–L5)
• Risk factors: increased body weight, cigarette smoking
• Pain worsened by sitting, standing, or maintaining a position for extended periods

4. Herniated (Prolapsed) Intervertebral Disc

• 90% of herniations occur at L5–S1 or L4–L5
• Triggered by flexion injuries or heavy lifting
• Posterior disc herniation → nerve root compression → radiculopathy/sciatica
• Large central herniation → cauda equina compression → bilateral pain, urinary retention, fecal incontinence
• Pain aggravated by bending, lifting, prolonged sitting, Valsalva (sneezing, coughing, straining); relieved by lying down

5. Spinal Stenosis

• A disease of advancing age
• Caused by: disc degeneration → reduced disc height → osteophyte formation + facet joint hypertrophy + ligamentum flavum hypertrophy/calcification → progressive narrowing of neural foramina and spinal canal
• Presents as neurogenic (pseudo)claudication: pain radiating to buttocks, thighs, legs — worse with exercise/standing, relieved by rest and spinal flexion ("shopping cart sign")
• Extensive osteophytes may compress multiple roots → bilateral pain
• Confirmed by MRI, CT, or myelography

6. Facet Joint Disease

• Degenerative changes in the zygapophyseal joints
• Pain near midline, radiating to gluteal region, thigh, and knee; associated with muscle spasm
• Worsened by hyperextension and lateral rotation
• Confirmed by pain relief with intra-articular local anesthetic or medial branch block

7. Congenital Abnormalities

• Often asymptomatic initially; abnormal spinal mechanics predispose to pain over time
• Includes:
  • Sacralization of L5 (fused to sacrum)
  • Lumbarization of S1 (acts as 6th lumbar vertebra)
  • Spondylolysis (pars interarticularis disruption)
  • Spondylolisthesis (anterior displacement of one vertebral body on another)
  • Spondyloptosis (subluxation of one body in front of the next)

8. Tumors

• Wide range of benign and malignant tumors can cause back pain
• Managed by neurosurgeons, radiotherapists, and/or oncologists

9. Infection

• Usually begins as discitis → progresses to osteomyelitis
• Causative organisms: pyogenic bacteria or Mycobacterium tuberculosis
• Chronic infection (e.g., spinal TB): back pain without fever or leukocytosis
• Acute infection (discitis, osteomyelitis, epidural abscess): acute pain, fever, leukocytosis, elevated ESR and CRP → immediate antibiotics; surgery if acute weakness develops

10. Arthritides

11. Referred Pain from Visceral / Retroperitoneal Structures

• Abdominal/pelvic pathology must always be considered, particularly:
  • Pancreas, kidneys, ureters, aorta (retroperitoneal)
  • Uterus and adnexa
  • Prostate
  • Rectosigmoid colon
• Disorders of the hip can also mimic back pain (positive Patrick's/FABERE test)

12. Neuropathic Components

• Chronic LBP and cancer pain may have prominent neuropathic components (paroxysmal, burning, lancinating quality with hyperpathia)
• Associated mechanisms: central sensitization, nerve root injury

Causes of Low Back Pain

Lumbar Disc Radiculopathy — Level Reference